Metal cans constitute a very widely utilized medium for the protective packaging of a great variety of products, many millions of cans being used daily for packaging of foods, beverages and many other materials. For many years, the common metal can was constructed of tin-plated steel, and was customarily referred to in the industry as an ETP can, the initials standing for "electro-tin-plated". Containers of this type are referred to by the public at large as "tin cans". In recent years, however, the metal can industry has developed organic polymeric resinous coatings for metal cans which offer substantially the same protection to the steel as the traditional thin coating of metallic tin. These organic resin based coatings have been applied both as a top coat over the traditional thin tin coating and as the sole protective coating composition applied directly to the steel can body and/or end components to yield both resin coated ETP cans and resin coated untinned steel cans which are now referred to in the industry as "tin-free steel" or TFS cans. This invention is directed primarily to ink compositions suitable for printing identifying indicia on the resin coated surfaces of TFS and coated ETP cans and can components by means of ink jet printing techniques.
Manufacturing and processing concerns which package various products in metal cans have found it highly desirable to print, at some point on the can surface, a series of coded symbols which carry information of interest primarily to the packager, including the particular machine on which the can was packed, the date and time of packing and perhaps even the identity of the machine operator. Such data are useful in case it is necessary to trace any particular can or cans after they have been packed.
Many of the products packaged in metal cans are subjected to conditions of high temperature and high moisture during pasteurization or sterilization procedures carried out before or after the can is filled with product and sealed. In order to be commercially satisfactory, the coded indicia printed on the cans must be capable of withstanding these processing conditions as well as being resistant to rubbing abrasion.
The invention of this application relates to ink compositions for use in a printing apparatus operating on the so-called "ink jet printing" principle. Ink jet printing is a recent development in the art of applying identifying and decorative indicia to a base. In general terms, a fluid ink is forced, under pressure, through a very small orifice in an orifice block which contains a piezoelectric crystal vibrating at high frequency (50-100,000 vibrations per second) causing the ink passing through the orifice to be broken into minute droplets equal in number to the crystal vibrations. The minute droplets are passed through a charging area where individual droplets receive an electrical charge in response to a video signal, the amplitude of the charge being dependent on the amplitude of the video signal. The droplets then pass through an electrical field of fixed intensity, causing a varied deflection of the individual droplets dependent on the intensity of the charge associated therewith, after which the deflected drops are allowed to impinge on the base medium which is to receive the decorative or informative printed indicia. Apparatus suitable for carrying out the ink jet printing process is described in detail in U.S. Pat. Nos. 3,465,350 and 3,465,351, issued Sept. 2, 1969, and it is in connection with an apparatus and process such as are described in the aforementioned patents that the ink of the present invention is designed to function.
In order to operate satisfactorily in an ink jet printing system, an ink must display a consistent breakup length, drop velocity and drop charge under set machine operating conditions. To achieve these ends, the ink must meet strict requirements with regard to viscosity and resistivity, solubility and compatibility of components, stability and anti-skinning properties and must readily re-dissolve in a suitable solvent for rapid cleanup of the machine components with a minimum of effort.
It has been determined that the workable range of viscosity of an ink which is to be used in a jet printing apparatus in which the nozzle orifice is 0.003 in. in diameter, must be no more than about 5 cps. at 68.degree. F., with about 1.90-2.0 being the most desirable viscosity level for superior performance. The viscosity may be somewhat higher than the above values if the orifice diameter is increased to 0.005 in., for example, but in any case an ink of less than 10 cps. and preferably less than about 5 cps. at 68.degree. F., is highly desirable. Resistivity may range from somewhat less than 100 ohm-cm. to about 1500 ohm-cm., the most desirable value being between about 150 and 300 ohm-cm. Resistivity in excess of about 1500 ohm-cm. creates problems in obtaining the proper charge on the droplets and therefore the deflectability of the droplets in an electric field is erratically impaired.
The orifice through which the ink must pass is normally in the range of 0.002 in. to 0.005 in. in diameter. In order to prevent plugging of this orifice, it is highly desirable that all components of the ink be in solution in the carrier medium rather than in a colloidal or other suspended state. In any case, the complete ink composition must pass at least a 2 micron filter in order to be satisfactory for use. Further, the ink components must not sludge out or otherwise deposit in any of the transporting lines, the supply tank, the orifice or any other portion of the ink supply system, even though the solvent medium of the ink is subject to a certain amount of evaporation in the ink return system and the supply tank. In other words, the solvent medium must have a reserve solubility for the solute components of the ink in order to prevent any undesired precipitation which could clog or plug the minute jet orifice. The ink must also possess anti-skinning properties to prevent skinning over of the orifice or the tank during periods of shut-down. Any skin formed in such circumstances could then break up into small solid particles which could plug the orifice.
In order to facilitate cleanup of the apparatus after use, the ink components should be readily soluble in a common solvent medium. This will prevent any gradual buildup of ink residues in the system which could result in malfunction.
The ink properties set forth above are primarily established by the requirements of the jet printing apparatus. In addition to these requirements, the ink must possess certain other properties which are specifically related to its intended use in the printing of metal cans and, in particular, coated steel can bodies intended for the packaging of foodstuffs and beverages.
For example, the ink must properly wet the coated can surface on which the printed indicia are to appear. If the ink is of such composition that it fails to readily wet the coated metal surface, the ink will bead up on the surface and fail to adhere properly to it. In extreme cases, the beaded drops will coalesce into larger droplets which run and make the printing completely unintelligible. The problem is often accentuated by oily or greasy residues left on the can surface from earlier stages of fabrication of the container. On the other hand, if the ink is of such composition as to wet the coated metal surface too readily, the ink drops will flatten out and spread by "crawling" on the surface, diluting the color intensity of the ink and overlapping the image of adjacent dots and spreading out sufficiently to make the printed image fuzzy and the characters unintelligible.
In addition to the requirement of proper wetting of the surface to be printed, the droplets of ink must adhere strongly to the surface after application and drying so that the printed matter is resistant to both physical rubbing or abrasive action and also is resistant to moisture. The ability of the ink to form and retain a desired image on a TFS or coated ETP can surface in the presence of moisture and the ability to resist removal by moisture is of great importance in this application because the metal can surfaces are generally damp before, during and after the printing operation. It is particularly difficult to maintain satisfactory adhesion of the ink to metal cans which are subjected to pasteurization, the combination of moisture and high temperature utilized in this process tending to cause the coloring matter to bleed, and to severely reduce the adherance of the ink to the can body so that it is readily removed by subsequent rubbing or abrasion.
It is an object of the present invention to provide ink compositions which will perform satisfactorily in ink jet printing apparatus to print on the surface of TFS cans identifying indicia which are resistant to abrasion and to the effects of the high temperature and moisture associated with steam pasteurization or sterilization processes. It is also an object of this invention to provide an ink suitable for ink jet printing on coated metal cans such as those used for the packaging of foods and beverages. It is a particular object of this invention to provide a jet printing ink for printing indicia on polymer resin coated metal cans which are to be subsequently submitted to pasteurization. Further objects will become evident from a consideration of the following specification and claims.
Most conventional fluid printing inks include three basic components. The first is a colorant for providing visibility of the printed indicia in contrast to the substrate surface. The colorant may be either a dye which is soluble in the ink solvent medium or a pigment which is suspended in the solvent medium. The second component is a resin or binder which remains on the substrate surface after printing and serves to adhere and bind the dye or pigment in position on the substrate surface. The third major component is the solvent which provides fluidity to the ink and carries in solution or suspension the resin and colorant. In addition to these three components which have heretofore been found in nearly all fluid printing inks, various other ingredients may be utilized, including drying, dispersing and wetting agents, plasticizers, diluents and the like. Ink jet printing inks, in addition to the three basic components above mentioned, may also desirably contain a fourth, optional component made appropriate to the ink composition by the nature of the apparatus and process of ink jet printing. This is an electrolyte, which is added so that the ink droplets may be given a strong, reproducibly variable electric charge which in turn enables a controlled, reproducible reflection of the droplets by application of an electrical field to the droplet stream. Printing processes other than ink jet printing do not require inks with electrical properties capable of achieving these ends.